Ultrasonic inspection of microscopic internal structures of semiconductor chips (integrating circuits: IC) and so on is carried out for the purpose of inspecting the bonding state of solders which join functional faces of the semiconductor chips and wiring substrates, and of fillers filled in gaps therebetween and for other purposes.
Methods of such inspection include a method in which an inspection object is irradiated with ultrasonic waves via water from a monocular ultrasonic transducer while the transducer is mechanically scan-moved in the water, the ultrasonic waves returning as echoes after being propagated through the inspection object are captured by the above-mentioned transducer, and signals thus obtained are processed, thereby judging the state of the inspection object.
What is becoming a problem at present in such an inspection method is that the underwater mechanical scan takes a lot of trouble and time so that, in the case of a semiconductor chip or the like, the chip becomes unusable after the inspection, and further, that lack of an abnormality judging capability may possibly be caused with the present inspection precision since the area and pitch of connection terminals of a semiconductor chip which is to be an inspection object are made narrower.
The ultrasonic transducer as stated above, an essential part of which can be manufactured by processing a piezoelectric material such as zinc oxide and tin oxide into a film, requires to have a certain film thickness in light of securing detection sensitivity. However, in a practical point of view, securing a large film thickness gives restriction on an upper limit of a frequency of a generated ultrasonic wave. This leads to lack of the abnormality judging capability since the frequency is substantially correlated with resolution.
A film thickness may be small when a piezoelectric material with a high sensitivity is selected, and, though it is understood that a resultant increase in drive frequency enables a high resolution to be obtained, it is generally difficult to achieve uniform formation.
Meanwhile, an ultrasonic imaging device having a monocular ultrasonic sensor transmits ultrasonic waves in a vertical direction using, for example, an immersion method, to perform imaging of a specific focal depth on the premise that a reflector exists in front thereof. In this case, when the inspection object has a curved surface, such a problem arises that imaging of an internal part of the inspection object is not possible, and for example, the focus is not fixed to prevent high-precision imaging.
Further, an ultrasonic imaging device having an ultrasonic transducer constituted of a number of piezoelectric elements arranged in a matrix or in a line can at least realize high precision, but a large amount of processing in inspecting and visualizing internal defects, voids, peel-off, and so on poses a problem when an inspection object has a layered structure having a plurality of different acoustic features and has a curved surface. Such processing requires two-dimensional or three-dimensional refraction calculation of the propagation of many ultrasonic waves transmitted/received among the piezoelectric elements which are arranged in a matrix, thereby resulting in an enormous amount of processing time.
This being the situation, for example, when the layered structure and the surface shape of the inspection object can be specified, such a method can be utilized in order to shorten the processing time that propagation time of ultrasonic signals transmitted/received among a number of the piezoelectric elements which are arranged in a matrix or in a line is calculated in advance according to propagation routes such as refraction or the like to store the resultant propagation time in a tabular form. This eliminates the necessity of conducting the refraction calculation each time. Still in this case, however, a sufficient calculation speed is not achieved due to a large number of the piezoelectric elements.
Meanwhile, in the ultrasonic inspection by the immersion method, a piezoelectric element (a transmitter and receiver of ultrasonic waves) and an object to be inspected are immersed in liquid such as water, and ultrasonic waves are transmitted/received while the piezoelectric element is mechanically scan-moved to visualize an inner part of the object to be inspected (refer to the following paper).
Ogura, “The Present Status of Non-destructive Inspection of Semiconductor Packages”, Non-destructive Inspection, Japanese Society for Non-Destructive Inspection, May 2001, Vol. 50, No. 5, p 291-292.
However, the piezoelectric element is immersed in the liquid in the immersion method so that durability thereof is liable to get low due to the intrusion of the liquid into the piezoelectric element. Moreover, it has been difficult to inspect the inner part of the object to be inspected disposed inside a hermetic container from an external part.